Propylene has grown in importance and it now has the second highest worldwide production rate in the petrochemical industry. Propylene has a variety of downstream uses such as polypropylene and propylene oxide and the demand is expected to grow at rates of 5% to 6% each year. Therefore, new, lower-cost routes to propylene production are of high interest in the petrochemical marketplace. Today the majority of propylene is produced either as a principal byproduct in steam cracking units, which primarily produce ethylene, or as a byproduct from fluid catalytic cracking units, which primarily produce gasoline. Both the steam cracker and the fluid catalytic cracking unit require naphtha as a feedstock to make significant quantities of propylene product although lesser amounts of propylene can be produced by feeding propane or butane to the steam cracking unit. While gasoline demand remains strong, ethylene demand is expected to grow at an annual rate of only 3% to 4%, below the growth rate in propylene. In addition, much of the new steam cracker capacity will be based on using ethane as a feedstock. Using ethane as a feedstock to a steam cracker typically produces only ethylene as a final product. While hydrocarbons heavier than ethylene are present, typically the quantities are too small to warrant recovery and there is essentially no propylene produced. These two factors, the higher growth rate of propylene product demand and the reduced quantity of co-produced propylene from new steam cracker units, have and will manifest themselves in a shortage of the supply of propylene and higher propylene product values.
The primary reason many of the new steam cracking units are using ethane as a feedstock is because ethane is a co-product of natural gas production and has limited value for uses other than as a feedstock to a steam cracker unit. As natural gas demand and production rates grow for supplying electrical power and home heating needs, ethane availability increases beyond its regional demand. Since ethane cannot be readily or economically transported, regional demand is important and where its availability exceeds regional demand, its price is reduced. In many regions, ethane feed costs are 25% to 50% of other steam cracker feedstocks such as propane, butane or naphtha. This gives a large advantage to producing ethylene using low cost ethane feedstock. In addition, energy costs and capital investments for a steam cracker using ethane feedstock are far below the costs for using propane, butane or naphtha as a feedstock.
One commercial operation does produce polymer grade propylene by producing butene from polymer grade ethylene in a dimerization plant and then using the formed butene to react with additional polymer grade ethylene to form the polymer grade propylene in a metathesis plant. However, that operation uses a liquid feedstock which is cracked to produce both ethylene and propylene with the propylene production only being supplemented by the conversion of polymer grade ethylene to propylene. That operation does not involve the cracking of a fresh ethane feedstock to produce ethylene and then using the ethylene in metathesis and dimerization reactions as the only source of propylene. The ability to produce propylene using ethane feedstock is currently not practiced commercially.
The ability to effectively produce propylene using ethane, or mostly ethane, feedstock would significantly lower feedstock costs, energy costs and required capital investment just as it does for the production of ethylene. Providing a route for propylene production from ethane feedstock would also better utilize the available ethane feedstock. As noted above, the only current utilization for the ethane co-produced in natural gas production is to use it as a feedstock for ethylene production which raises concerns about an oversupply of ethylene product and the impact that would have on ethylene pricing and demand. Hence, some producers do not utilize all the available ethane. Providing a route to produce propylene from ethane feedstocks would help ensure that ethane utilization is high in all regions.